1. Field of the Invention
The subject invention relates generally to the field of control apparatus and more particularly to control apparatus for use in multiple steam generator or multiple hot water generator installations.
2. Prior Art
Many steam or hot water generating facilities utilize a single generator having sufficient capacity to satisfy the maximum predicted loads. In most instances, the actual steam/hot water load is much less than the maximum load; therefore the single generator will be operating at an inefficiently low firing rate. In order to increase efficiency, multiple medium and multiple low capacity generators have been used. During high steam/hot water load conditions, all of the generators will be on line to meet the increased demand. At lower loads, one or more of the generators will be placed on standby or will be completely shut off so that the remaining generators will be fired at higher and generally more efficient firing rates.
Various control apparatus have been used for optimizing the efficiency of multiple steam/hot water generator facilities. One such prior art control apparatus is used for controlling facilities utilizing forced flow type generators which are capable of being brought on line from a shut off condition to a maximum firing rate in typically five minutes. In most applications, the generators are brought on and off line in response to the measured steam pressure or measured hot water temperature which are indicative of the steam load or hot water load, respectively.
Operation of such prior art control apparatus can be described briefly as follows. At high steam pressures or water temperatures, it is possible that only one generator will be on line with the remaining generators being shut off. Should the steam pressure or water temperature drop a predetermined amount, a second generator will be brought on line. In the event the pressure/temperaure further drops another predetermined amount, a third generator will be brought on line (if one is available). This process will continue until the steam pressure or water temperaure stabilize or until all available generators have been brought on line. Similarly, should the steam pressure or water temperature increase, generators will be taken off line in a staged manner until either the pressure or temperature has either stabilized or until there is only a single generator remaining on line.
A generator which is shut off consumes significantly less energy than a generator which is placed on standby or at a minimum firing rate. Thus, the above-described prior art control apparatus significantly improves the efficiency of a system in comparison to control apparatus wherein the off-line generators are placed on standby. However, such control apparatus possesses significant limitations, especially in applications wherein the steam pressure or water temperature must be maintained relatively constant. As previously noted, a generator is brought on line in response to a detected drop in steam pressure or water temperature which indicates that the steam or hot water load has increased. If a forced flow type generator is used, there will be a typically worst case five minute delay before an off line generator can be brought on line and brought up to maximum firing rate (assuming that the drop in steam pressure is sufficiently great to require a maximum firing rate). If a conventional fire tube (drum) boiler is used, the delay could be an hour or more. During this delay, the steam pressure or water temperature will continue to drop below the nominal value. Accordingly, the steam pressure or water temperature will deviate significantly from the desired nominal value.
A second shortcoming of the above-described prior art control apparatus becomes apparent in situations wherein the steam/hot water load momentarily decreases thereby causing a short duration increase in steam pressure or water temperature. Ideally, the temporary decrease in load would be ignored and all generators on line would remain on line. However, the prior art control apparatus will detect the drop in load and may cause one or more generators to be taken off line. When the temporary decrease in steam or hot water load is over, the steam pressure will drop, or the temperature will drop so that the generators which were taken off line will be again needed. However, the pressure or temperature will remain significantly below the desired nominal value until the generator has been brought back on line.
The present invention overcomes the above-described limitations of the prior art controllers. Rather than rely solely upon the measured load for controlling the generators, apparatus is provided for predicting anticipated changes in loads. By way of example, if an increase in load is predicted, one or more generators can be brought on line prior to such increase. Thus, the generators will be able to meet the increase in demand in a relatively short period of time (typically thirty seconds) so that the steam pressure or water temperature will not deviate significantly from the desired nominal value. Similarly, should there be an unanticipated temporary decrease in load, the present invention will cause the generators to remain on line. Accordingly, when the steam or hot water load returns to normal, the generators will be able to rapidly meeet the increase in load without any significant drop in steam pressure or water temperature.
The present invention possesses the above-described advantages over the prior art in addition to having further advantages as will be apparent from the following detailed description.